In processes employed in chronic blood-cleansing therapy, such as hemodialysis, hemofiltration, and hemodiafiltration, blood is conveyed through an extra-corporeal blood circuit. As an access to the blood vessel system, an arteriovenous fistula is often made by an operation. Also possible is the use of an implant. When a “fistula” is mentioned below, what is meant by this is any kind of connection between a vein and an artery of the patient.
The blood flowing through the fistula is only used during the actual dialysis treatment. In the period during which there is no dialysis, the flow of blood in the fistula is equivalent to a functional left/right shunt in which a proportion of the arterial blood produced by the cardiac output is fed directly to the venous system and the heart and does not undergo any peripheral use. The fistular flow recirculates via the heart and lungs. The fractional proportion which the fistular flow represents of the cardiac output is defined as cardio-pulmonary recirculation. The cardio-pulmonary recirculation affects not only the load on the patient's circulation but also the efficiency of the dialysis. Because the dialyzed blood from the extra-corporeal circuit is mixed with the venous return from the greater circulation while bypassing the systemic areas of the circulation, there is a systematic reduction in the concentration of dialyzable constituents in the arterial blood (See D. Schneditz et al.: Cardiopulmonary recirculation during hemodialysis. Kidney Int. 42: 1450-1456, 1992).
For the ability of the fistula to work properly, its perfusion is an important factor. If the fistular flow drops below a critical level, there is an increased risk of a fistular thrombosis and the possible loss of the vascular access, which constitutes a major complication in dialysis treatment (See W. Bay et al.: Color Doppler flow predicts PTFE graft failure, J. Am. Soc. Nephrol. 5: 407 (1994)). If the fistular flow during the dialysis treatment is less than the extra-corporeal blood flow (QB), local fistular recirculation occurs, in which a fraction of the dialyzed blood which is fed back to the fistula by the venous blood line is fed back to the dialyser via the arterial blood line. The fistular recirculation (RA) causes a significant reduction in the efficiency of dialysis (See F. Gotch: “Models to predict recirculation and its effect on treatment time in single-needle dialysis” First Intl. Symposium on Single-Needle Dialysis, Editors: S. Rignoir, R. Vanholder and P. Ivanovich, Cleveland, ISAO Press, 1984, page 305 ff). Measurement of the quality of the vascular access is thus an important way of ensuring quality in dialysis treatment.
Because of its clinical importance, there are various known methods of measuring recirculation. Common to all of them is the measurement of a physical or chemical parameter of the blood which is changed in the venous segment of the extra-corporeal circuit. The physical or chemical parameter of the blood can be changed by manually injecting an indicator or even indirectly via the dialysis treatment unit.
Where there is mention below of recirculation (R), fistular recirculation (RA) or the cardio-pulmonary proportion of recirculation (RCP), what these terms should be understood to mean are not absolute quantities but the proportions which the respective types of recirculation represent of the cardiac output. In practice, relative quantities are enough to enable the recirculation processes taking place in the fistula to be assessed.
A method of measuring recirculation which is called thermodilution is known from the EDTNA-ERCA Journal 19, 6 (1993). In this known method, a brief drop in temperature is initiated in the dialysis-fluid circuit and this is transmitted to the venous segment of the extra-corporeal circuit and produces a detectable sudden change in temperature in the arterial segment of the extra-corporeal circuit when recirculation is occurring.
A known arrangement for carrying out the method called thermodilution has a temperature sensor arranged in the arterial segment of the extra-corporeal circuit and a temperature sensor arranged in its venous segment. The venous temperature sensor senses the sudden change in temperature which is attributable to the drop in temperature caused in the dialysis-fluid circuit. The sudden change in temperature which is measured is analyzed and is then compared with the temperature curve registered at the arterial sensor. The ratio of the two integrals for temperature to one another, or the ratio between their amplitudes, is a measure of the overall reduction in the efficiency of the dialysis treatment caused by fistular and cardio-pulmonary recirculation.
The known arrangement for measuring recirculation has proved successful in practice. However, something that has been found to be a disadvantage is that it is only possible to measure the total recirculation, which will be referred to below as recirculation (R), which is the sum of fistular recirculation (RA) and a proportion which derives from the cardio-pulmonary recirculation, which will be referred to below as the cardio-pulmonary proportion of recirculation (RCP). A distinction must be made in this case between the cardio-pulmonary proportion of recirculation (RCP) and the proportion of fistular flow in the cardiac output, which will be referred to below as the cardio-pulmonary recirculation (Rcp).
That method of measuring the total recirculation made up of fistular recirculation and cardio-pulmonary recirculation which is referred to as thermodilution is also described in Drukker, Parsons and Maher, Replacement of Renal Function by Dialysis, 5th edition, 2004, Kluwer Academic Publishers BV.
A method of measuring recirculation (R), i.e., the sum of fistular recirculation (RA) and the cardio-pulmonary proportion of recirculation (RCP), is known from German Patent Publication No. DE 197 02 441 C1. In this known method, a physical or chemical parameter of the dialysis fluid is changed, along the path followed by the dialysis fluid, upstream of the dialyzer, and this causes a change in the physical or chemical parameter on the blood side. The change in the parameter of the dialysis fluid on the blood side results in a change in the parameter of the dialysis fluid downstream of the dialysis-fluid chamber of the dialyzer. To determine recirculation, the parameter on the path followed by the dialysis fluid is measured downstream of the dialyzer and recirculation (R) is determined from the curve followed over time by the change in the parameter. What may be changed and measured as a physical or chemical parameter is the ion concentration in the dialysis fluid, such example as the Na concentration in the dialysis fluid, or even the temperature of the dialysis fluid. However, something that is disadvantageous is, once again, that what can be determined by the known method is not fistular recirculation or cardio-pulmonary recirculation but only recirculation as a whole.
German Patent Publication No. DE 195 28 907 C1 describes a method of determining cardio-pulmonary recirculation. The measurement of cardio-pulmonary recirculation is based on two measurements of the recirculation fraction which follow closely on one another and which are carried out automatically before and after the reversal of the blood flow. It is a disadvantage that this known method calls for the blood flow to be reversed.
U.S. Pat. No. 6,537,240 B2 describes a method of determining recirculation in which the ultrafiltration rate is changed and before and after the change in the ultrafiltration rate a value is determined for a blood parameter which is representative of the ratio of the volume of blood plasma to the volume of blood.
An object underlying the present invention is to specify a method which allows fistular recirculation and/or cardio-pulmonary recirculation to be determined during extra-corporeal blood treatment without the flow of blood in the extra-corporeal blood circuit being reversed.
A further object of the present invention is to provide an arrangement for determining fistular recirculation and/or cardio-pulmonary recirculation without the flow of blood being reversed.
Furthermore, it is an object of the present invention to provide a blood-treating apparatus which allows fistular recirculation and/or cardio-pulmonary recirculation to be determined without the flow of blood being reversed.